Depth or range cameras are devices that produce a 2D image showing the distance between the sensor device and positions (e.g. points) in a scene. The resulting image, the range image, has pixel values which correspond to the distances to these positions. If the sensor is properly calibrated, the pixel values can be given directly in physical distance units such as meters.
A time-of-flight camera (ToF camera) is a range imaging camera that determines the distance based on the time needed for a light signal to travel from the camera to an object in the scene, and back, taking into account the speed of light (c=approximately 300,000 km per second). The camera has an illumination unit, typically implemented with one or more lasers or LEDs, which is switched on for a very short time, the resulting light pulse illuminates the scene and is reflected by objects present therein. The camera gathers the reflected light onto a sensor plane. Depending on the distance, the incoming light experiences a delay with respect to the emitted light pulse. An object 1.0 m away will delay the light by 6.66 ns (from the camera to the object and back to the sensor). This example makes clear that any additional delay, even as small as 10 ps, will result in distance measurement errors. Several techniques exist for measuring this delay, and some kind of calibration is usually required to compensate for any unwanted delays of the sensor; however, such calibration is typically done only during production of the camera.
A problem associated with range sensors is that internal delays may vary over time or manufacturing tolerances, e.g., but not limited thereto, due to temperature drift and/or ageing effects and/or external lighting conditions, causing measurement errors. In typical environmental conditions and under typical manufacturing tolerances, internal delays could vary over a range of more than 100 ps. In typical applications, 1 cm or better range accuracy is required, hence requiring delay tuning over a wide range (several 100 ps) and with high resolution (10 ps). Such internal delay variations may be compensated by dedicated hardware elements, with the drawback of increased components, increased complexity and thus cost of such sensors.
A time-of-flight camera is described by Schwarte in WO98/10255, wherein an optical feedback path from a light emitting module to one or more sensor cells of the light receiving camera is used as a phase reference for synchronization purposes, by guiding the emitted light without reflection to the receiver, e.g. through an optical fibre.
A very comprehensive description of the time-of-flight camera described in WO98/10255 is disclosed by IEE in U.S.2008180650(A1), and incorporated hereafter to illustrate the workings of a ToF camera. Referring to FIG. 1, the block diagram of the 3D imaging system 10 comprises an illumination unit 12 for emitting light onto a target scene, and an imaging sensor 14, for imaging the target scene. The imaging sensor 14 comprises the required optical accessories such as a focusing lens (not shown) and an electronic camera chip executed in any suitable technology, such as CCD (charge coupled device), CMOS (Complementary Metal Oxide Semiconductor) and/or TFA (thin film on ASIC). Accordingly, the imaging sensor 14 comprises a two-dimensional array of individual lock-in pixel sensor cells 16 each of which images a small portion of a target scene for creating a pixel-by-pixel image thereof. The illumination unit 12 comprises several individual light emitting devices 18 such as light emitting diodes (LEDs), which are collectively driven by means of an illumination driver 20. A signal source 22 provides the input signals for the illumination driver 20 and a photo gate driver 24. The output of the photo gate driver 24 is connected to the imaging sensor 14. An evaluation unit 26 comprising a suitable electronic calculation device, e.g. a digital signal processor (DSP), is connected to the output of the imaging sensor 14.
In operation, the 3D imaging system 10, based on the phase shift measurement method, works as summarized below. The signal source 22 generates a modulation signal E1 on its output and feeds this modulation signal E1 to the illumination driver 20. The latter drives the illumination unit 12 with a drive signal E2 to emit a temporally modulated light signal L1 onto a target scene comprising an object 30 (for illustration purposes). Examples for temporal modulation of the light signal L1 are a sinusoidal amplitude (i.e. light intensity) modulation or a periodically pulsed emission scheme. The modulated light signal L1 is reflected or scattered by the object 30 so as to form a returning light signal L2 which is received as incident light by the imaging sensor 14. The modulation signal E1 is also fed to the photo gate driver 24 which transforms the modulation signal E1 into a demodulation signal E3. The imaging sensor 14 receives this demodulation signal E3. By means of this demodulation signal E3 and the detected light signal L2, the imaging sensor 14 produces a phase (and optionally, amplitude) information signal E4 which is fed to the evaluation unit 26 for extraction of the distance information concerning the object 30. Further details regarding the 3D imaging technology schematically shown in FIG. 1 can be found e.g. in WO98/10255 and the relevant literature.
The measurement accuracy of the device shown in FIG. 1 is limited due to unknown and/or time varying signal propagation times and component delays. In fact, ideally there should be no phase difference between the light signal L1 emitted at the illumination unit 12 and the demodulation signal E3. However, an uncontrollable phase difference may be introduced between these signals due to several factors such as time varying delays in the illumination driver 20 and the photo gate driver 24, e.g. due to temperature or ageing. This phase difference adversely affects synchronization. As a result, significant errors may occur in the determination of the absolute distance information, which is based on the phase relationship between the light signal L1 emitted at the illumination unit 12 and the light signal L2 detected at the imaging sensor 14. The approach taken by Schwarte in WO98/10255 to address this problem requires extra hardware, such as a light conductor and optical shielding.
U.S.2008180650(A1) further discloses a 3D-imaging system comparable to the one described in WO98/10255, but wherein the optical feedback path is replaced by an electrical feedback path. Such a system still requires additional hardware, such as one or more elements selected from the group of a shunt resistor, a mixer and an optical element.